The present invention relates to the fabrication of integrated circuits. More particularly, the invention provides a technique, including a method and apparatus, for depositing tungsten (W) films.
Deposition of tungsten over a semiconductor substrate is a common step in the formation of some integrated circuit (IC) structures. For example, tungsten deposited by a chemical vapor deposition (CVD) technique is commonly used to provide electrical contact to portions of a semiconductor substrate. These electrical contacts are usually provided through openings in an insulation layer, such as a silicon oxide layer, formed over the substrate. One common vertical interconnect film stack includes a titanium (Ti) film for the contact, followed by a thin titanium nitride (TiN) diffusion barrier film, and a thicker CVD tungsten film to fill the majority of the via and serve as the conducting film. Tungsten CVD provides excellent step coverage, particularly in high aspect ratio (HAR) vias, and a high deposition rate for the deposited tungsten film.
One CVD technique that has been employed to deposit tungsten films in the semiconductor industry uses tungsten hexafluoride (WF6) and a hydrogen reducing agent, as precursor gases. One known process that uses this deposition technique includes two main steps: nucleation and bulk deposition. The nucleation step grows a thin layer of silane-reduced (SiH4) tungsten which acts as a growth site for subsequent film. In addition to WF6 and SiH4, the process gas used in the nucleation step may include molecular hydrogen (H2), nitrogen (N2) and argon. A bulk deposition step is then used to form the tungsten film from a process gas including WF6, H2, N2 and Ar.
As advances in integrated circuit technology lead to a scaling down of device dimensions and an increase in chip size and complexity, issues which were not previously considered problematic are becoming a concern. Once such issue is fluorine contamination during the tungsten deposition process. Fluorine contamination in a tungsten via is a consequence when WF6 is used for tungsten CVD. Fluorine contamination in the film stack is undesirable due its"" ability to increase resistivity of the films and its"" reactivity with other elements, particularly titanium. Fluorine can also diffuse out of the via and into the transistor device during subsequent thermal processing. Increased fluorine levels in the gate oxide have been correlated with increased penetration of boron dopant in the gate oxide and channel regions with negative implications for device performance.
One method of reducing fluorine contamination in the titanium film is by increasing the thickness of the TiN diffusion barrier. However, this approach increases resistance in the via because of the higher resistivity of TiN relative to tungsten. Also, the TiN film may have reduced step coverage relative to tungsten CVD meaning a thicker TiN film may result in lower overall step coverage in the via. The continuing decrease in via diameter has made the use of thinner, not thicker, diffusion barriers desirable.
Accordingly, improvements in the deposition of tungsten films that result in reduced fluorine contamination are desirable.
The present invention provides a method and apparatus for forming an improved tungsten film having reduced fluorine contamination film. Embodiments of the invention result in minimal fluorine penetration into underlying liner/barrier films and can be used for via applications requiring void free, 100% step coverage.
One embodiment of the method of the present invention includes a chemical vapor deposition process for depositing a tungsten film on a substrate disposed in a substrate processing chamber. This embodiment includes depositing a first layer of the tungsten film by flowing a first process gas comprising tungsten hexafluoride, silane, molecular hydrogen and argon into the substrate processing chamber, where a flow ratio of the molecular hydrogen to argon is at least 1.5:1 and the partial pressure of tungsten hexafluoride is less than or equal to 0.5 Torr. The second layer of the tungsten film is then deposited over the first layer by flowing a second process gas comprising tungsten hexafluoride and a reduction agent into the substrate processing chamber.
These and other embodiments of the present invention, as well as its advantages and features are described in more detail in conjunction with the text below and attached figures.